NO328462B1 - Image projection system with polarization beam parts - Google Patents

Description

This invention relates to an image projection system for the visible spectrum and which comprises a polarization beam splitter which reflects one linear polarization of light and allows the other polarization of light to pass. In particular, the invention relates to such a system with a compact, lightweight beam splitter which comprises several elongated, reflective elements which act in such a way when electromagnetic waves from a source light are applied that they generally emit or allow one light polarization to pass while the other light polarization is reflected.

Polarized light is necessary in some applications, such as projection in liquid crystal displays (LCD). Such a display typically comprises a light source, optical elements, such as lenses for collecting the light, a first polarizer that sends a first polarization of the light to the crystals, a group often called an array of such crystals to modify the light polarization for encoding the image information, means for addressing each picture element in the array to either change or retain its polarization, a second polarizer (called an analyzer) to reject the unwanted light from the selected picture elements, and a screen on which the image is focused.

It is possible to use one and the same polarization beam splitter (PBS) as both the first and the second polarizer. If the crystal array is reflective, such as a liquid crystal on silicon (LCOS) fluorescent tube, it can reflect the beam coming from the polarizer directly back to it after encoding the image by modifying the polarization of selected picture elements. Such a system was proposed by Takanashi (US patent 5,239,322). The idea was further developed by Fritz and Gold (US 5,513,023). These similar methods provided important advantages in optical layout and performance. However, none of these have been realized in practice due to the disadvantages associated with conventional polarization beam splitters. The disadvantages of using conventional polarization beam splitters in projection displays include images that are not bright, have poor contrast and non-uniform color balance or intensity (due to non-uniform performance across the light cone). In addition, many conventional polarization beam splitters have a short life due to excessive heat and are also very expensive.

For such an image polarization system to be commercially successful, it must be able to deliver images significantly better than those from conventional cathode ray tube (CRT) television tubes, due to the fact that such a system is likely to be more expensive than conventional conventional CRT technology. Therefore, the image polarization system must deliver (1) clear images with corresponding colors or color balance, (2) have good image contrast and (3) be as affordable as possible. An improved polarization beam splitter (PBS) represents an important step in achieving this goal since the PBS is a limiting component that determines the potential performance of the display system.

The PBS properties that significantly affect display performance are (1) the angular aperture or f-factor at which polarization can be maintained, (2) the absorption or energy losses associated with the use of PBS, and (3) the PBS lifetime. In optics, the angular opening or f-factor describes the angle of the light cone at which the PBS can use and maintain the desired performance. Larger cones or smaller f-factor are desirable because larger cones allow more light to be collected from the light source, leading to greater energy efficiency and more compact systems.

The absorption and energy losses associated with the use of PBS naturally affect the system's brightness, since the more light that is lost in the optics, the less light is left that can be projected on the screen. In addition, the amount of light energy absorbed by the polarizer will affect its lifetime, especially in such image projection systems where the light passing through the optical system is very intense with several watts per square centimeter. Light of such intensity can easily damage ordinary polarizers, such as Polaroid plates. In fact, the lifetime limits polarizers that can be used in such applications.

The lifespan is also important, because the smaller and lighter the projection systems can be produced, the less expensive and more desirable the product becomes. In order to achieve this, however, the light intensity must be increased even more and further strain the PBS and thus shorten its lifetime.

A problem with conventional PBS devices is poor conversion efficiency which is the most important, critical performance factor of displays. Conversion efficiency vi teten is a measure that describes how much electrical power is required by the light source, which is translated into light intensity power on the screen or panel that is visible to the public. This is expressed as the ratio between the total light output on the screen divided by the electrical output required by the light source. Conventional units are lumens per watt. A higher ratio is desirable for several reasons, for example, a low conversion efficiency will require a more powerful light source, with the consequent greater power supply, excess heat, larger cabinets, etc. In addition, all these consequences of low conversion efficiency can increase the costs of the projection system.

A fundamental cause of low conversion efficiency is poor optical efficiency, which is directly related to the f-factor or f-number of the optical system. A system that has an f-number that is half the f-number of an otherwise equivalent system has the potential to be four times as efficient in gathering light from a light source. Consequently, it is desirable to provide an improved polarization beam splitter (PBS) which makes it possible to utilize the light energy better by being able to offer a significantly smaller, potential f-number (larger angular opening) and consequently increase the conversion efficiency, measured in lumens/watt.

There are several reasons for the poor performance of conventional polarization beamsplitters in terms of conversion efficiency when used as beamsplitters in projection systems. First, current beam splitters perform poorly if the light source does not reach them at a certain angle (or at least, one within a narrow cone of angle around this main angle of incidence). The deviation of the main beam from this angle will cause each type of polarization beam splitter to decrease in intensity and weaken the polarization, and/or the color balance. This applies to the beam coming from the light source as well as the beam reflected from the liquid crystal screen. This principal angle depends on the design and construction of the PBS as well as the physics of the polarization mechanism used in these different beam splitters. Current, available beam splitters are not able to operate efficiently at angles far from their main polarization angles in the visible part of the electromagnetic spectrum. This limitation makes it possible to implement certain promising optical arrangements and conventionally promising display designs.

Although the main beam hits the polarizer at the best angle to separate the two polarizations, other beams cannot deviate far from this angle before the visual qualities are degraded. This is a serious shortcoming in display equipment because the light hitting the polarizer must be highly converging or diverging to make effective use of the light emitted from typical light sources. This is usually expressed in the f-number in the optical system. For a single lens, the f-number is the ratio of the aperture to the focal length. For optical elements in general, the f-number is defined as

F/# = 1/(2 n sin d)

where n is the refractive index in the space in which the optical element is placed and # is half the cone angle. The smaller the f-number, the greater the radiating flux <j>c which is captured by the lens and the more efficient the device will be to display a bright image. The refraction flux increases as the inverse square of F/#. In an optical array, the optical element with the largest F/# will be the limiting factor in the optical efficiency. For displays using traditional polarizers, the limiting element is almost always the polarizer and thus PBS limits the conversion efficiency. It would therefore be an advantage to be able to develop a type of PBS that has a smaller F/# than what exists today.

Because traditional small F/# polarizers have not been available, developers have typically attempted to increase conversion efficiency by specifying a smaller, brighter light source. Such sources typically have arc lamps, but they require expensive power supply units that are both heavy and large and require constant cooling during operation. Cooling fans cause unwanted noise and vibration. This makes projectors and similar display equipment unsuitable. Again, a PBS with a small F/# will be able to effectively collect light from a low efficiency, quiet, conventional light source.

Other important disadvantages of conventional polarization beam splitters are low extinction, which leads to poor contrast in the image. Extinction is the ratio of light transmitted through the polarizer of undesired polarization to the light rejected by undesired polarization. In an effective display, this ratio must be maintained at a minimum value over the entire cone of light passing through the PBS. Therefore, it is desirable to provide a polarization beam splitter that has a high extinction level leading to a high contrast image.

A third disadvantage of conventional polarization beam splitters is a non-uniform response across the entire visible spectrum, or poor color fidelity. The result is poor color balance which leads to further inefficiency in the projection display system as light from strong colors must be rejected to accommodate the weaknesses in the polarization beam splitter. Accordingly, it is desirable to provide an improved polarization beam splitter which has a uniform response over the entire visible spectrum (or good color fidelity) and which renders an image with good color balance with better efficiency. The beam splitter must be achromatic rather than degrading the projected colors and must not allow overlap between the polarizations, as this will degrade image sharpness and contrast. These properties must apply over all parts of the polarization and over all angles of incident light occurring at the polarizers. The term "spatial" has been used (R.C. Jones, Journ. Optical Soc. Amer. 39, 1058, 1949) to describe a polarizer that preserves overlapping areas, solid angle, and the relative intensity distribution of wavelengths in the polarized beam. A PBS that serves as both polarizer and analyzer must be spasmatic for both transmission and reflection, even in light beams with a large angle aperture.

A fourth disadvantage of conventional polarization beam splitters is the short lifetime. Many conventional polarization beam splitters suffer from degradation caused by excessive heat and photochemical reactions. Accordingly, it is desirable to provide an improved polarization beam splitter which can withstand an intense photon flux for several thousand hours without showing signs of deterioration. In addition, it is desirable to provide a polarization beam splitter that can be produced economically on a large scale.

The need to meet these and other criteria has led to only a few types of polarizers for practical use in projection systems. Many attempts have been made to include both a wide angle aperture and a high fidelity polarization in the same beam splitter device. The results of these efforts are described below. Thin film noise filters are the type of polarizer that is mentioned most often to produce a polarization beam splitter that can also be used as an analyzer. MacNeille was the first to describe such a polarizer that was effective over the entire spectral range (US Patent No. 2,403,731). It consists of several layers of thin film that are diagonal to the incident light, typically inside a glass cube, which makes it large and heavy compared to the plate polarizer. Furthermore, it must be designed from a single angle of incidence, typically 45°, and its performance is poor if the incident light deviates from this angle of incidence by as much as 2°.

Others have improved the design (eg, J. Mouchart, J. Begel and E. Duda, Applied Optics 25, 2847-2853, 1989; and L. Li and J.A. Dobrowolski, Applied Optics 13, 2221-2225, 1996). All of these found it necessary to significantly reduce the wavelength range if the angular opening was to be increased. This can be done in some designs (US Patent 5,658,060 and 5,798,819, where the optical design divides the light into suitable color bands before it arrives at the polarization beam splitter. In this way, it is possible to reduce the spectral bandwidth requirements of the beam splitter and expand its angular opening, however, the additional components and complexity significantly increase the cost, size and weight of the system.

Nevertheless, these improved beam splitter cubes came on the market and are available from well-known distributors, such as Balzers and OCLI. They can offer an F/# of f/2.5 - f/2.8 which is a significant improvement over what was available just two years ago, but still far from the range of F/1.2 - F /2.0 which is completely within other important components of optical projection systems. If these f-numbers are reached, system efficiency can be improved by as much as a factor of 4. It also makes it possible to make previously impossible design compromises to achieve other objectives, for example reduced physical size and weight, lower costs, etc.

In a technology other than visible optics, namely radar, wire gratings have been used successfully to polarize long radar wavelengths.

These wire grid polarizers have also been used as reflectors. They are also known in optical components for infrared (IR) light, where they are used mainly as polarization elements for the transmission.

Although it has not been demonstrated, some have suggested the use of a wire grid polarizer in displays in the visible part of the spectrum. For example, Grinberg (US Patent 4,688,897) proposed that a wire-grid polarizer could serve as both a reflector and an electrode (but not simultaneously as an analyzer) for a liquid crystal display.

Others have suggested the possible use of a wire grating polarizer instead of a dichroic polarizer to improve efficiency in virtual image displays (see US 5,383,053). However, the need for contrast or slope in the grating polarizer is omitted and the grating is used mainly as a polarization-sensitive beam steering device. It does not serve to be used as an analyzer or polarizer in patent document 5 383 053. It also appears from the text that a broadband polarizing cube beam splitter would have been able to perform the same service if this had been available. However, this technology is rejected for being too limited in terms of approved angle or even function, while being far too expensive.

Another patent (US Patent 4,679,910) describes the use of a grating polarizer in an imaging system designed for testing IR cameras and other IR instruments. In this case, the application requires a beam splitter for long-wavelength infrared where a grating polarizer is the only practical solution. The patent does not suggest the use of the visible area or mention the need for a large angle opening. Nor does it suggest the need for efficient conversion of light into a visible image or the need for broadband performance.

Other patents also exist for wire grating polarizers in the infrared part of the spectrum (US Patents 4,514,479, 4,743,093 and 5,177,635, for example). With the exception of the previously mentioned exceptions, emphasis is placed on the transmission performance of the polarizer in the IR spectrum.

These references show that it has been known for many years that wire gratings can function generally as polarizers. Nevertheless, they have obviously not been proposed and developed for image projection systems. One possible reason why wire grid polarizers have not been used in the visible spectrum is manufacturing difficulties. US patent 4,514,479 describes a method for holographic exposure of photoresist and subsequent etching in an ion plant to produce a wire grating polarizer for the near infrared region. In US Patent 5,122,907, small, elongated ellipses of metal are baked into a transparent matrix which is then stretched to conform the metal ellipses to their long axes, to some degree. Even if the projected beam is polarized, the device does not reflect well enough. Furthermore, the elliptical particles have not been made sufficiently small to be used in the visible part of the electromagnetic spectrum.

Consequently, practical applications have generally been limited to the longer wavelengths in the IR spectrum.

Another prior art polarizer achieves much finer lines by near-angle evaporative deposition (US Patent 4,456,515). Unfortunately, the lines have such small cross-sections that the interaction with the visible light is weak and thus the optical efficiency is too poor for use in the production of images. As in several of these known techniques, this device has threads with shapes and spaces that are largely random. Such random placement weakens the performance because areas with nearby elements do not transmit well enough and areas with wider placed elements have poor reflectivity. The resulting degree of polarization (quenching) will be less than maximum if one or both of these things occur, which is quite certain if the placement is so arbitrary.

For perfect (and near perfect) regularity, the math developed for the lattice also applies. Conversely, the scattering theory for arbitrary threads (even if they have the same direction) gives the best description. Scattering from a single, cylindrical wire has been described (H.C. Van de Hulst, Light Scattering by Small Particles, Dover, 1981). Current random thread couplers have threads baked into the substrate. Not only is the position of the threads arbitrary, but so are the diameters. It is clear that the phases between the scattered rays will be random, so that the reflection will not be strictly specular and that the transition will not retain its high spatial or image fidelity. An attenuation of the light beam will prevent its use for transmission of high-resolution, high-density images.

Nothing in the prior art indicates or suggests that an ordered array of filaments can or should be used over the entire visible range as a spatic PBS, at least at the angles required when serving as both a polarizer and an analyzer. The difficulty of producing the narrow, tall and evenly spaced threads required for such use has been described quite extensively (see Zeitner et al. Applied Optics, 38, 11, pp. 2177-2181 (1999) and Schnabel et al. .Optical Engineering 18, 2, pp. 220-226 (1999)). Consequently, it is not surprising that the prior art for image projection does not suggest the use of a spatic PBS as part of a display device.

Tamada and Marsumoto (US patent 5,748,368) describe a wire grid polarizer that works both in the infrared and part of the visible spectrum. However, it is based on the idea that large, widely spaced wires will create resonance and polarization in an unexpectedly short wavelength in the visible range. However, this device only works well over a narrow band of visible wavelengths and not over the entire visible spectrum. It is therefore not suitable for use in producing images in all colours. Accordingly, such a device is not practical for image projection, due to the fact that a polarizer must be substantially achromatic for an image projection system.

Another reason why wire grid polarizers have been overlooked is the common and persistent assumption that the performance of a typical wire grid polarizer is degraded as the beam incidence angle of the light becomes large (G.R. Bird and M. Parrish Jr., "The Wire Grid as a Near-Infrared Polarizer" , J. Opt. Soc. Am., 50, pp. 886-891 (1960), Handbook of Optics, Michael Bass, vol. II, pp. 3-34, McGraw-Hill (1995)). There are no reports of structures that will work well at angles of incidence greater than 35° in the visible part of the spectrum. Nor has anyone demonstrated important design factors that cause this limitation of the angle of incidence. This limited perception of design becomes greater when one discovers that a successful beam splitter will deliver good performance in both transmission and reflection at the same time.

This important point deserves attention. The extensive literature and patent history for wire grating polarizers in the IR and visible spectrum has focused almost entirely on their use as a transmission polarizer and not on their reflective properties. Wire-grid polarizers have been tried and reported in the technical literature for many decades and have become increasingly common since the 1960s. Despite the extensive work that has been done in this area, there has been very little, if any, detailed discussion of the fabrication and use of wire grating polarizers as reflective polarizers and nothing in the literature regarding their use as both transmission and reflection polarizers at the same time, which would be necessary in a spatial polarization beam splitter for use in image display devices. Based on the lack of discussion in the literature, a sober examiner will come to the conclusion that a potential use of wire grating polarizers as broadband, visible beam splitters is not prominent, or that it was commonly understood in the technical field that their use in such an application was not practical.

Since the conventional polarizers described above were the only ones available, it was impossible for Takanashi (US Patent No. 5,239,322) to reduce his projection apparatus to perform all but the most reasonable results. No polarizer was available that provided the performance required for the Takanashi invention, namely achromatism over the visible part of the spectrum, wide-angle matching, low transmission and reflection losses of the desired light polarizations, and a good extinction ratio.

There are several important features of an image display system that require specialized performance for transmission and reflection. From a projector, the product of p-polarization transmission and s-polarization reflection (RsTP) must be large if a source light is to be effectively placed on the screen. On the other hand, it is important for the resolution and contrast needed to achieve a high information density on the screen, that the above-mentioned product

(RPTS) is very small (ie the transmission of s-polarized light multiplied by the reflection of p-polarized light must be small).

Another important feature is a wide approval angle. The acceptance angle must be large if the light collected from the light source, and therefore the conversion efficiency, is to be maximized. It is desirable that the light cone (either diverging or converging) with half angles greater than 20° can be approved.

An important consequence of the ability to accept larger light cones and work well within larger angles is that the optical design of the imaging system is no longer limited. Conventional light sources can then be used with their advantages of low cost, cool operation, small size and weight. A wide variety of angles can enable the designer to position the other optical elements in favorable positions to improve the display's size and operation.

Another important feature is size and weight. The conventional technology requires the use of a glass cube. This cube involves certain requirements and loads in the system. The requirements include the need to handle thermal load on this large piece of glass and the need for high quality materials without burdensome birefringence etc., which entails additional costs. In addition, the extra weight and size of the cube itself means difficulties. Thus, it is desirable that the beam splitter does not take up too much space and does not weigh too much.

Another important feature is the solvency. Modern light sources generate very high thermal rises in the polarizer immediately after the light is switched on. At best, this can cause thermal bipolarization which can cause switching between the polarizations. Furthermore, the long duration of exposure to intense light can cause some materials to change their properties (typical yellowing from photooxidation). Thus, it is desirable that the beam splitter can withstand high temperatures as well as long periods of intense irradiation from a light source.

Yet another important feature is uniform extinction (or contrast) of the beam splitter across the incident cone of light. A McNeille-type thin film stack polarizer produces polarized light due to the difference in reflectivity in S-polarized light as opposed to p-polarized light. Since the definition of S and P polarization depends on the plane of incidence of the light beam which changes direction within the cone of light incident on the polarizer, the McNeille polarizer does not work equally well across the cone. This weakness of McNeille polarizers is well known. The weakness must be resolved in projection systems by limiting the angular size of the light cone and by compensating elsewhere in the optical system using additional optical components. This fundamental weakness of the McNeille prisms increases the cost and complexity of current projection systems and limits system performance through limitations in the f-number or beam splitter optical efficiency.

Other important features include adaptability. Production costs and maintenance are both directly affected by the assembly criteria. These costs can be significantly reduced with components that do not require low-tolerance fittings.

Consequently, it would be advantageous to develop an image projection system that can clear images and have good image contrast and that is not expensive. It would also be advantageous to develop an image projection system with a polarization beam splitter that can utilize divergent light (or have a smaller F/# ratio), that can make efficient use of the light energy or that has a high conversion efficiency, and that is also durable. It would also be advantageous to be able to develop an image projection system with a polarization beam splitter which has a high extinction ratio, uniform response over the entire visible spectrum, good color fidelity, which is spatic, robust and which can withstand thermal rises. It would also be advantageous to be able to develop an image projection system with a polarization beam splitter which can be positioned at essentially any angle of incidence, so that significant constructional obstacles do not arise on the image projection system, but which allows a large degree of construction flexibility. It would also be advantageous to be able to develop an image projection system with a polarization beam splitter that effectively transmits p-polarized light and reflects s-polarized light over all angles in the entire incident light cone. It would also be advantageous to be able to develop an image projection system with a polarization beam splitter that is light in weight and compact. It would also be advantageous to develop an image projection system with a polarization beam splitter that is easy to adapt. Combining all these features in a single projection device will involve a significant advance in the technique.

It is therefore an object of the invention to provide an image projection system which provides clear images with good image contrast and which is affordable.

It is further an object of the invention to provide an image projection system with a polarization beam splitter that utilizes divergent light (or has a smaller F/# ratio), efficiently uses the light energy, has good conversion efficiency and is durable.

It is a further object of the invention to provide an image projection system with a polarization beam splitter with a high extinction ratio, a uniform response over the entire visible spectrum, good color fidelity, which is spatic, robust and which receives thermal rises.

It is a further object of the invention to provide an image projection system with a polarization beam splitter which can selectively direct each or both of the transmitted and reflected polarized beams at substantially any angle.

It is a further object of the invention to provide an image projection system with a polarization beam splitter which functions satisfactorily when it is positioned in relation to the light source beam at essentially any angle of incidence.

It is further an object of the invention to provide an image projection system with a polarization beam splitter which effectively transmits p-polarized light and reflects s-polarized light over all angles within the light source, but which can also function to transmit s-polarized light and reflect p-polarized lights at the same time.

It is a further object of the invention to provide an image projection system with a polarization beam splitter which is light in weight, compact, robust and which is easy to adapt.

It is a further object of the invention to provide a polarization beam splitter for use in an image projection system.

These and other objects and advantages of the invention are realized in an image projection system with a polarization beam splitter which is preferably a wire grid polarizer. The beam splitter with wire grating polarization has a generally parallel arrangement of thin, elongated elements. The arrangement is configured and the elements are adapted to interact with electromagnetic waves of the source light beam to generally transmit one polarization of light through the elements and reflect the other polarization from the elements. Light with a polarization that is aligned perpendicular to a plane that includes at least one of the elements and the direction of the incident light is transmitted and forms a transmitted beam. The opposite polarization, or light with a polarization that is aligned parallel to the plane comprising at least one of the elements and the direction of the incident light beam, is reflected and forms a reflected beam.

The system includes a light source to produce a mostly internally polarized, visible light beam. The polarization beam splitter is placed close to the light source in the light beam. The system also includes a number of reflective floating crystals. The array may be located close to the polarization beam splitter in either the reflected or transmitted beam. The array modulates the polarization of the beam and produces a modulated beam. The array is arranged to direct the modulated beam to the beam splitter. The arrangement of elements in the beam splitter interacts with electromagnetic waves in the modulated beam to again generally transmit one polarization and reflect the other. Thus, the reflected part of the modulated beam defines a second, reflected beam, while the transmitted part forms the second, transmitted beam. The array changes the polarization of the beam to encode image information on the modulated beam. The beam splitter separates the modulated polarization from the unmodulated beam and thus produces the image that is visible on a screen.

A screen is placed either in the second, reflected, second transmitted beam. If the array is located in the reflected beam, the screen will be located in the other, transmitted beam. If the array is located in the transmitted beam, the screen will be located in the second, reflected beam.

In contrast to previous large, heavy beam splitters, the beam splitter according to the invention is a generally flat plate. The beam splitter is also efficient and provides greater light efficiency in the system.

According to one aspect of the invention, the beam splitter is capable of being aligned in relation to the light beam and the modulated beam at incident angles between approximately 0 and 80°.

In accordance with yet another aspect of the invention, the light beam has a useful directing cone with a half angle between about 12 and 25°. The beam splitter is used at a small F number, preferably between approximately 1.2 and 2.5.

According to another aspect of the invention, the beam splitter has a conversion efficiency of at least 50% defined by the product of the s-polarization of reflected light and the p-polarization of transmitted light (R/Tp). In addition, the s-polarized, transmitted light and the p-polarized, reflected light are both less than 5%. Furthermore, the proportion of reflected light and the proportion of transmitted light in the modulated beam is greater than approximately 67%.

According to another aspect of the invention, the system may comprise a pre-polarizer placed between the light source and the beam splitter and/or a post-polarizer placed between the beam splitter and the screen.

The aforementioned objects are achieved with an image projection system as stated in the introduction, which has the characteristic features as stated in claim 1. The method for projecting an image has the characteristic features as stated in claim 13. Preferred embodiments of the invention are stated in the independent claims .

In the following, the invention will be described in more detail in connection with design examples and with reference to the drawing figures, where fig. 1A is a schematic diagram of the general operation of a preferred embodiment of an image projection system according to the invention employing a wire grating polarization beam splitter, FIG. 1B and 1C are schematic views of the image projection system according to the invention in different configurations, fig. 2A is a graphical representation showing the relationship between wavelength and the transmittance for both the S and P polarizations of a preferred embodiment of the wire grid polarization beam splitter according to the invention, fig. 2B is a graphical representation showing the relationship between wavelength and the reflectance for both the S and P polarizations in a preferred embodiment of the wire grid polarization beam splitter according to the invention, fig. 2C is a graphical representation showing the relationship between wavelength and efficiency and transmission extinction of a preferred embodiment of the wire grating polarization beam splitter according to the invention, FIG. 3 is a graphical representation showing the performance of a preferred embodiment of the wire grating polarization beam splitter according to the invention, as a function of the angle of incidence, fig. 4A is a graphical representation showing the theoretical performance of an alternative embodiment of the wire grating polarization beam splitter according to the invention, FIG. 4B is a graphical representation showing the theoretical extinguishing performance in an alternative embodiment of the wire grid polarization beam splitter according to the invention, fig. 4C is a graphical representation showing the theoretical extinguishing performance in an alternative embodiment of the wire grid polarization beam splitter according to the invention, fig. 5A is a schematic view of the general operation of an alternative embodiment of an image projection system according to the invention, fig. 5B and 5C are schematic views of the image projection system according to the invention in different configurations, fig. 6 is a schematic diagram of the general operation of an alternative embodiment of the image projection system according to the invention, fig. 7 is a perspective view of the wire grating polarization beam splitter according to the invention, fig. 8 is a side section of the wire grating polarization beam splitter according to the invention.

Reference will now be made to the drawings where the various elements of the invention will be given numerical designations and where the invention will be described so that a person skilled in the art can carry out and use the invention.

As shown in fig. IA is a display optical array of an image projection system according to the invention designated generally at 10. The image projection system 10 preferably has a wire grating polarizer as a beam splitter, designated generally at 14. The wire grating polarization beam splitter 14 (WGP-PBS) effectively reflects light of one polarization from a source 20 to a series of floating crystals 26 and then efficiently transmits the reflected light of opposite polarization to a display screen 25.

For sufficient optical efficiency, the WGP-PBS 14 must have a high reflectivity (Rs) of the desired polarization from the light source 20 and must have a high transmissivity (TP) of the opposite polarization from the crystal array 26. The conversion efficiency is proportional to the product of these two, RsTP, so that a deficiency in one factor can be compensated to a certain extent by an improvement in the other.

Examples of grating polarization beam splitters 14 according to the invention preferably show the following properties which show the advantages of using a WGP-PBS 14 according to the invention both as polarizer and analyzer in display devices for the visible part of the spectrum. Theoretical calculations of further improvements show that better polarization beam splitters will also be available.

In fig. 2A and 2B show the measured transmittance and reflectivity for both S and P polarizations of a WGP-PBS. In fig. 2C, the efficiency of WGP-PBS is shown as the product of transmittance and reflectivity. In addition, the extinguishing is also shown in fig. 2C. In fig. 2A-2C, the WGP-PBS is arranged to reflect the S-polarization and transmit the P-polarization at incidence angles of 30°, 45°, and 60°. In an image projection system, such as a projector, the product of a reflected S-polarization and transmitted P-polarization (RsTp) must be greater if the light source is to be placed effectively on the screen. For the necessary resolution for high information density on the screen, on the other hand, it is important that the conversion product (RpTs) is very small (i.e. that the transmission of S-polarized light multiplied by the reflection of P-polarized light must be small) . It will be seen from the figures that the wire grating polarization beam splitter according to the invention meets these standards over the entire spectrum without impairment due to Rayleigh resonance or other phenomena.

Another important feature is a wide approval angle. This must be large if light collected from the light source, and consequently the conversion efficiency, is to be maximized. In fig. 3 shows the performance of a wire grating polarization beam splitter according to the invention for different parts of the light cone centered around the optical axis with an angle of incidence of 45°. In fig. 3, a first reference angle is the angle in the plane of incidence, while the second reference angle is the angle in the plane perpendicular to the plane of incidence. It will be seen that the WGP-PBS according to the invention can accept light cones (either divergent or converging) with half angles between approximately 12 and 25°.

In fig. 4A-4C, theoretical calculations for an alternative embodiment of the wire grating polarization beam splitter indicate that significantly larger light cones and/or other improvements are possible. Fig. 4A and 4B show the theoretical result of quenching in a wire grating polarization beam splitter with a period p reduced to 130 nm. In addition, the grating height or thickness is 130 nm, the line spacing ratio is 0.48, the substrate groove depth is 50 nm, and the substrate is BK7, glass. It will appear from fig. 4A that the result is grouped closer than the result shown in fig. 2C. Consequently, the performance can be improved by reducing the period P. It will also appear from fig. 4B that the quenching is significantly increased compared to fig. 2C.

Fig. 4C shows the theoretical quenching in yet another alternative embodiment of the wire grating polarization beam splitter with the period p further reduced. The wavelength is 420 nm and the angle of incidence is 30°. It should be noted that the quenching increases markedly as the period p is reduced.

As shown above, an important consequence of the ability to accept larger light cones with a WGP-PBS, which will work well at wide angles, is that the PBS no longer limits the optical design of the image display system. Thus, conventional light sources can be used with the advantage of lower costs, cooler operation, smaller size and light weight. The large range of angles over which the WGP-PBS will work allows the other optical elements to be positioned in favorable positions to improve the display's size and use. In fig. 1B and 1C, the design flexibility brought about by the wide range of angles in the PBS according to the invention is shown. As shown in fig. IB, the light source 20 and the row 26 can be placed closer together, as both have a relatively small angle of incidence in relation to the PBS 14. Such a configuration is advantageous for achieving a compact design of the components in the system 10. Alternatively, and as shown in fig. 1C, the light source 20 and the row 26 can be placed further apart, since both have a relatively large angle of incidence. It should be noted that in each case the angles of incidence vary greatly from the 45° angle typically required in traditional beam splitters.

Other properties of wire mesh provide advantages for display units. Conventional technology requires the use of a glass cube. This cube entails certain requirements and disadvantages for the system. The requirements include the need to cope with thermal stress in this large piece of glass, the need for high-quality materials without burdensome birefringence, etc., which entails additional costs and greater weight and bulk of the cube itself. The WGP-PBS according to the invention is advantageously a divided or patterned thin film that does not take up much space and does not weigh much. It can also be integrated with, or included in, other optical elements such as color filters to further reduce the number of parts, the weight and the scope of the projection system.

The WGP-PBS according to the invention is also very robust. Modern light sources generate very high thermal rises in the polarizer immediately after the light is switched on. At best, this can cause thermal and strain double-focusing that causes overlap between the polarizations. At worst, it can delaminate the multilayer polarizer or cause the cemented junction in the cube beam splitter to break apart. Furthermore, the long duration of exposure to intense light causes some materials to change their properties (typically yellowing due to photooxidation). However, wire grating polarization beam splitters are made of a chemically inert metal that adheres well to glass or other substrates. These have proven to be able to withstand high temperatures as well as long periods of intense irradiation from the light source.

The WGP-PBS according to the invention is also very easy to adapt. There is a single part that must be adjusted to direct the source beam at the row of floating crystals. This is the same simple procedure used for a flat mirror. There is also another adjustment parameter, namely the angular rotation about the normal of the WGP surface. This determines the orientation of the polarization in the light beam. This adjustment is not critical because WGP acts as its own analyzer and cannot get out of adjustment in this sense. If there are other polarization elements in the optical array, the WGP-PBS should be aligned with respect to their polarization, but slightly out of alignment is not critical because: according to Malus' law, an angular variation makes very little difference in the intensity transmitted by the polarizer if their polarization axes are close to being parallel (or perpendicular).

In order to compete with conventional polarizers, the product RsTP must be submerged by approximately 50%. This represents a lower calculation that is only practical if the WGP-PBS can collect significantly more light from the light source than conventional polarization beam splitters. The 50% calculation, from an assumption that the best conventional beam splitter, a modern McNeille cube beam splitter, can deliver an f/# of f/2.5, at best. An optical system that was twice as fast, or that could collect twice as much light, would then have an f/# of 1/V2 for this value, or f/1.8 which is a reasonable f/# in optical image projection systems. A system that is twice as fast, and therefore can collect twice as much light from the source, will roughly compensate for the factor of 2 reduction in the RgTP product over the conventional cube beam splitter and lead to an equivalent projection system performance. Since WGP-PBS can potentially actually be used down below fl 1.2 (a factor of 4 increase), this apparently low limit will still produce very bright images. Naturally, an RsTP product that is above this minimum value will give an even better performance.

Another important factor is the contrast in the image, as defined by the ratio between the light intensity and dark image elements. One of the important advantages of WGP-PBS is improved contrast over all angles of incidence compared to previous cube beam splitters, such as the McNeille prism. The McNeille prism produces light by taking advantage of the difference in reflectivity in the S v P polarization at certain angles. Since the S and P polarization are defined relative to the plane of incidence, the effective S and P polarization from a particular beam in a light cone relative to the beam along the optical axis varies among different rays within the light cone. The consequence of this behavior is the well-known collection angle problem where the extinction of the polarizer is significantly reduced for certain angular areas within the light cone that pass through the polarization beam splitter, which significantly reduces the average contrast over the cone.

WGP-PBS, on the other hand, uses a different physical mechanism to achieve polarization of light, which to a large extent avoids this problem. This difference in behavior is due to the fact that the polarization is caused by wire gratings in the beam splitter which have the same arrangement in space regardless of the plane of incidence of a particular beam in the light cone. Although the plane of incidence for a particular beam is the same when incident into a McNeille prism or a WGP, the polarization effect is only dependent on the plane of incidence in the case of the McNeille prism, this would imply that the overall angular performance in the WGP is greatly improved over that provided by the cube beam splitter.

The fact that the function of WGP-PBS is independent of the plane of incidence means that WGP-PBS can actually be used with threads or elements aligned in any direction. The preferred embodiment of the invention has the elements aligned parallel to the axis around which the polarizer is tilted, so that the light falls on the WGP-PBS at an angle. This arrangement is preferable since it causes the polarization effects of the surface reflections from the substrate to be added to the polarization effects from the grating. However, it is possible to produce a WGP-PBS that can function to reflect the P polarization and transmit the S polarization (which is exactly the opposite of what has generally been described herein) over certain ranges of the angles of incidence by rotating the grating elements, as that they are perpendicular to the inclination of the WGP-PBS axis. Likewise, the grating elements can be placed at an arbitrary angle in relation to the tilt axis to obtain a WGP-PBS that can transmit and reflect light with polarizations adapted to the projection of this arbitrary angle on the light beam's wavefront. It will therefore appear that WGP-PBS which reflects the P polarization and transmits the S polarization, or which reflects and transmits light with the polarization aligned at an arbitrary angle, is covered by this invention.

The overall angular performance advantage according to WGP-PBS gives a more uniform contrast over the entire light cone and is one of the reasons why WGP is suitable for very small f-numbers. But of course this is not the only factor that affects image contrast. The image contrast is controlled to a large extent by a small leakage of unwanted polarization, but in this case the product TsRp is not the only parameter due to the image generation sequence that lies in the sequence after the first encounter with the beam splitter, but before the second, and which takes place in the production of the image contrast. Consequently, the final system contrast will depend on the fluorescent tube performance as well as the polarization quenching. However, lower limits for the necessary beam splitter performance can be determined based on the assumption that the fluorescent tube performance is sufficient for it to be assumed to have substantially unlimited contrast. In this case, the system contrast will depend entirely on the beam splitter performance.

In fig. IA there are two different functions fulfilled by the beam splitter 14. The first is the preparation of the polarized light before it reaches the row 26 of floating crystals, or other suitable image generating device. The requirement here is that the light is sufficiently well polarized, so that variations in the polarization of the light beam can be produced by the light tube and sufficiently detected or analyzed, so that the final image will meet the desired performance level. Likewise, the beam splitter 14 must have a sufficient performance to be able to analyze the light which is directed by the light tube back to the beam splitter, so that the desired system contrast is achieved.

These lower limit values can be easily determined. In terms of applicability and image quality, however, it is doubtful that an image with a contrast of less than 10:1 (light image element in relation to nearby dark image element) will have greater applicability. Such a display will not be suitable for dense text, for example. If a minimum display system contrast of 10:1 is assumed, an incident light cone that has at least ten times the light of the desired polarization relative to the undesired polarization would be required. As for the polarization performance, it can be described as having a closure of 10:1 or simply 10.

The second encounter with the beam splitter 14, which is to analyze the image, must be able to pass the light to the right polarization state, which eliminates most of the light from the unwanted state. Assuming a light beam with an image encoded in the polarization state and that this light beam has a 10:1 ratio, a beam splitter that maintains this 10:1 ratio would be desirable to meet the goal of a 10:1 system contrast. In other words, it is desirable to reduce the light from the unwanted polarization by a factor of 10 in relation to the right polarization. This in turn leads to a minimal extinguishing performance of 10:1 for the beam splitter's analysis function.

Higher system contrast will naturally occur if one or both of the polarization and analysis functions in the beam splitter have a higher quenching performance. It is also clear that it is not required that the performance in both the analyzer function and the polarizer function in the beam splitter are adapted for an image projection system to work satisfactorily. An upper limit of the beam splitter's polarization and analysis function is difficult to determine, but it is clear that quenches above about 20,000 are not necessary for this purpose. A high-quality film projection system, such as in a cinema, typically does not have an image contrast above 1000 and it is doubtful that the human eye can distinguish between an image with a contrast in the range of several thousand and a contrast of 10,000. If it is necessary to produce an image with a contrast of several thousand and assuming that the fluorescent tubes can handle this, an upper limit for the beam splitter cut-off of around 10,000 to 20,000 will be sufficient.

The above outline of the minimum and maximum limits of the wire mesh beam splitter is indicative, but it will be apparent from what is shown and the theoretical performance of a wire mesh beam splitter, as shown above, that much better than this can be achieved. According to this information, the preferred embodiment has RsTP >65% and RP or Ts or both >67%, as shown in Fig. 2A-2C. The preferred embodiment will also use the wire grating polarization beamsplitter in the mode where the reflected beam is directed towards the image generating array which guides the light back to the beamsplitter so that it passes through, or is sent through, the beamsplitter. This preferred embodiment is shown in fig. IA.

As shown in the image display system 60 of FIG. 5A, alternatively, the wire grating polarization beam splitter 14 can effectively transmit light of one polarization from the source 20 to the array of reflective floating crystals and then effectively reflect the reflected light of the opposite polarization to the display screen 25. The second embodiment of the image projection system 60 is similar to that shown in the preferred embodiment in fig. IA, with the exception that the beamsplitter 14 will be used in a manner where the source beam of light is sent or passed through the beamsplitter 14 and directed towards the image generating array 26 and then reflected back to the beamsplitter 14 where it is reflected by the beamsplitter and analyzed before being displayed on screen 25.

In fig. 5B and 5C, the design flexibility from the wide range of angles in PBS, according to the invention, is shown. As shown in fig. 5B, the row 26 and the screen 25 can be placed closer together, as both have a relatively small angle of incidence in relation to the PBS 14. As shown in fig. 5C, the row 26 and the screen 25 can alternatively be placed further apart, since both then have a relatively large angle of incidence.

As shown in fig. 6, a third embodiment of the image projection system 80 provides an alternative system design that can help achieve the desired level of system performance. This third embodiment would include one or more additional transmitting or reflective polarizers operating in series with the wire grating polarization beam splitter to increase quenching in one or both of the polarization and analysis functions to achieve the desired system contrast. Another reason for additional polarizers would be the implementation of a polarization recovery arrangement to increase system efficiency. A pre-polarizer 82 is placed in the source light beam between the light source 20 and the WGP-PBS 14. A post-polarizer or cleaning polarizer 84 is placed in the modulated beam, or the beam reflected from the array 26, between the array 26 and the screen 25, or between the WGP-PBS 14 and the screen 25. The third embodiment will still achieve the advantages of the wire mesh beam splitter's larger light cone, durability and the other advantages mentioned above.

As shown in the figures, the image display system can also use the light collecting optics 90 and the projection optics 92.

In fig. 7 and 8, the wire grating polarization beam splitter 14 according to the invention is shown in detail. The polarizing beam splitter is further discussed in detail in co-filed US application 09/390,833, filed September 7, 1999, entitled "Polarizing Beam Splitter", which is incorporated herein by reference.

As described in the concurrently filed application, the polarization beam splitter 14 has a grating 30, or a series of parallel, conductive elements, placed on a substrate 40. The source light beam 130 produced by the light source 20 is incident on the polarization beam splitter 14 with the optical axis at an angle 6 from the normal, where the plane of incidence is preferably perpendicular to the conducting elements. An alternative embodiment will place the plane of incidence at an angle 6 in relation to the plane of the conducting elements, where 6 is approximately 45°. In another, alternative embodiment, the plane of incidence will be parallel to the conducting elements. The polarization beam splitter 14 divides this beam 130 into a mirror-reflected component 140 and a transmitted component 150. Using the standard definitions for S- and P-polarization, the light with S-polarization has the polarization vector perpendicular to the plane of incidence and thus parallel to the conducting elements. Conversely, the light with P polarization has the polarization vector parallel to the plane of incidence and thus perpendicular to the conducting elements.

Ideally, the polarization beam splitter 14 will act as a perfect mirror for S-polarized light and will be perfectly transparent for the P-polarized light. In practice, even the most reflective metals used as mirrors will absorb a fraction of the incident light and thus WGP will reflect only 90% to 95% and ordinary glass will not transmit 100% of the incident light, due to the reflection in the surface.

The most important physical parameters of the wire grid beam splitter 14 that must be optimized as a group to achieve the desired performance include: the period p of the wire grid 30, the height or thickness t of the grid elements 30, the width w of the grid elements 30, and the slope of the grid element sides. It will appear from fig. 8 that the general cross-section of the grid elements 30 is trapezoidal or rectangular. This general shape is also a necessary feature of the polarization beam splitter 14 according to a preferred embodiment, but tolerances are given for the natural, small variations due to the manufacturing methods, for example the rounding of the corners 50 and the hollow wedges 54 in the bottom of the grating elements 30.

It should also be noted that the period p of the wire grating 30 must be rectangular in order to achieve the desired specular reflection required to meet the image fidelity requirements of the beam splitter 14. While it is natural to have the grating 30 completely rectangular and uniform, in some applications it can be relaxed the requirements, as these are not absolutely critical. However, it is assumed that a variation in the period p of less than 10% over the meaningful dimension in the image (eg, as the size of a single character in a text display, or a few image elements in an image) is required to achieve the desired performance .

Likewise, reasonable variations in the beam splitter 14 of the other parameters described, for example the width w of the grating elements 30, the height of the grating element t, the side slopes or the rounding of the corners 50 and the hollow wedges 54 are also possible without the display performance being significantly affected, especially if the beam splitter 14 is not a image plane in the optical system, as is often the case. These variations may also be visible in the final beam parts 14, such as fringing, variations in transmission efficiency, reflection efficiency, color uniformity, etc. and still be a useful part in specific applications of the projection imaging system.

The design goal that must be met by the optimization of these parameters is to produce the best efficiency or result possible, while at the same time meeting the requirements for contrast for use. As mentioned above, the smallest practical quench required by the polarization beam splitter 14 is of the order of 10. It has been found that the minimum required output (RsTP) of the beamsplitter 14, to obtain a valuable product, is about 50%, which implies that one or both of Rp and Ts must be above approximately 67%. Naturally, higher performance in both the result and the quenching of the beam splitter can be of value and provide a better product. To understand how these parameters affect the performance of the wire mesh beam splitter, it is necessary to examine the variation in performance produced by each parameter at an angle of incidence of 45° and also possibly other angles of interest.

The performance of the wire grating beamsplitter 14 is a function of the period p. The period p of the wire grating elements 30 must fall below about 0.21 µm to produce a beamsplitter 14 of reasonable performance throughout the visible spectrum, although it will be clear to one skilled in the art that a larger period in the beam splitter will be applicable in the system that is expected to display less than the entire visible spectrum, eg only red, red and green, etc.

The performance of the wire mesh beam splitter 14 is a function of the height or thickness of the element. The wire mesh height t must be between approximately 0.04 and 0.5 µm to achieve the desired performance.

The performance of the wire grating beam splitter 14 is a function of the width and period ratio (w/p) of the elements 30. The width w of the wire element 30 relative to the period p must fall within the range of approximately 0.3 to 0.76 to achieve the desired performance.

The performance of the wire mesh beam splitter 14 is a function of the side slope of the elements 30. The slope of the sides of the wire elements 30 is preferably greater than 68° relative to horizontal to achieve the desired performance.

It will be clear that the described embodiments of the invention are only for illustration purposes and that modifications can therefore be carried out by a person skilled in the art. For example, placement of the wire grid beam splitter on a substrate with optical power supply, for example so that the wire beam splitter is combined in relation to prior art, can be achieved by the invention. Accordingly, the invention is not considered to be limited to the described embodiments, but only by the appended claims.

or integrated with other elements to reduce the number of optical elements required, system weight, system size, or to achieve other desired properties. Other changes will certainly be apparent to one skilled in the art in view of the significant increase in design flexibility

Claims (14)

1. Image projection system (10, 60, 80) for providing an image, characterized by: a light source (20) which produces a visible light beam (130), a polarization beam splitter (14) placed near the light source, in the light beam from it and aligned at an angle relative to it, the light source comprising: a transparent substrate (40) with a first surface positioned in the light beam so that it is incident obliquely thereon, and a generally parallel arrangement of thin, elongated elements (30) carried by the substrate, the arrangement is configured and the elements are adapted to interact with electromagnetic waves in the source light beam to generally i) transmit the light through the elements having a polarization aligned perpendicular to the plane comprising at least one of the elements and the direction of the incident light, forming an emitted beam (150), and ii) reflect the light from the elements having a polarization aligned parallel to the plane comprising at least one of the elements and the direction of the incident incident light beam, forming a reflected beam (140), a reflected array (26) being located near the polarization beam splitter in either the reflected or transmitted beam, the array modulating the polarization of the beam by selectively changing the polarization of the beam to encode the image information thereon and produce a modulated beam, the array being arranged to direct the modulated beam back towards the polarization beam splitter, the beam splitter being further placed in the modulated beam and arranged at an angle to the modulated beam and the arrangement of elements in the beam splitter working together with electromagnetic waves in the modulated beam to generally i) transmit light through the elements having a polarization aligned perpendicular to the plane encompassing at least one of the elements and the direction of the incident light beam, forming a second transmitted beam, and ii) reflect the light from the elements which has a polarization that is parallel to the plane that encompasses the least one of the elements and the direction of the incident light beam, forming a second reflected beam, to separate the unchanged polarization from the modulated beam, and a screen (25) placed in either the second reflected beam or the second transmitted beam to display the encoded image information. 2. System according to claim 1, characterized in that the beam splitter is a generally flat plate. 3. System according to claim 1, characterized in that the beam splitter is aligned in relation to the light beam or the modulated beam at an angle of incidence between 0 and 80°. 4. System according to claim 1, characterized in that the beam splitter is aligned in relation to the light beam or the modulated beam at an angle of incidence that is greater than 47° or less than 43°. 5. System according to claim 1, characterized in that the beam splitter is used with an f-number of less than f/2.5. 6. System according to claim 1, characterized in that the beam splitter has a capacity of at least 50% as defined by the product of the fraction of the p-polarization of transmitted light and the fraction of s-polarized, reflected light, and where the s-polarized, transmitted light and p-polarized reflected light are both less than 5%. 7. System according to claim 1, characterized in that the beam splitter has a capacity for the light beam of less than 65%, as defined by the product of the fraction of reflected light and the fraction of transmitted light, and where the proportion of reflected light or the proportion of transmitted light is greater than 67%. 8. System according to claim 1, characterized in that it further comprises a pre-polarizer (82) placed between the light source and the beam splitter. 9. System according to claim 1, characterized in that it further comprises an after-polarizer (84) placed between the beam splitter and the screen. 10. System according to claim 1, characterized in that the arrangement of elements has a period that is less than 0.21 [im, as the elements have a thickness of 0.04 and 0.5 (im, and as the elements have a width of between 30 and 76% of the period. 11. System according to claim 1, characterized in that the elements each have a cross section in relation to a bottom, a top opposite the bottom and opposite left and right sides, and where the sides form an angle in relation to the bottom that is greater than 68°. 12. System according to claim 1, characterized in that the light source has a wavelength between 0.4 and 0.7 µm. 13. Method for projecting an image, the method being characterized by: producing a source light beam (130) with a wavelength in the range between 0.4 and 0.7 (im by means of a light source (20), substantially separating the polarizations of the source light beam by means of a polarization beam splitter (14) located in the source light beam, the polarization beam splitter comprising a generally parallel arrangement of thin elongated elements (30) configured and adapted to interact with electromagnetic waves in the source light beam to generally i) pass the light beam through the elements having a polarization which is aligned perpendicular to a plane that includes at least one of the elements and the direction from the incident light beam, forming a transmitted beam (150), and ii) reflect the light from the elements that have a polarization aligned that lies in the plane that includes at least one of the elements and the direction of the incident light beam, forming a reflected beam (140), modulate either d a transmitted or reflected beam and producing a modulated beam by selectively changing the polarization of the beam by means of an array (26) disposed in either the transmitted or reflected beam, substantially separating the polarizations of the modulated beam by means of the polarization beam splitter disposed in the modulated beam , the elements acting together with electromagnetic waves of the modulated beam to generally i) transmit light through the elements having a polarization aligned perpendicular to the plane comprising at least one of the elements and the direction of the incident light beam, forming a second transmitted beam , and ii) reflect light from the elements having a polarization lying in the plane encompassing at least one of the elements and the direction of the incident light beam, forming a second, reflected beam, and presenting either the second transmitted beam or the second reflected beam on the screen (25). 14. System according to claim 1, characterized in that the light source has a wavelength in the range between 0.4 and 0.7 (im, and the beam splitter is aligned in relation to the source light beam at an angle of incidence between 10 and 80°.